Numerical Study on Flow Stability Improvement of an Automotive HVAC Compressor’s Expansion Chamber Using Parametric Mufflers

Pressure pulsation and mass flow-rate fluctuation of the refrigerant are major flow-side excitation sources of noise and vibration in automotive HVAC systems, which make flow stability inside the compressor an important design concern. In this paper, flow characteristics of working fluids are analyzed based on the numerical model of a mass-produced automotive HVAC compressor’s expansion chamber. A parametrically configurable muffling structure is introduced, and its geometric parameters are varied one at a time, with the remaining parameters held at fixed values, to evaluate their influence on the flow field. Among the single muffler models examined, the best-performing one yields a reduction of 34.32% on pressure pulsation, and 53.31% on mass flow-rate fluctuation, compared to the original model. Subsequently, a new design of a double muffling structure is proposed to further improve the flow stability. A total of 25 distinct models are compared, and the best-performing models under two criteria, minimum pressure pulsation and minimum mass flow-rate fluctuation, are different. The structure that yields minimum pressure pulsation value achieves a reduction of 49.73% on pressure pulsation and 61.58% on mass flow-rate fluctuation, compared to the original model, while the one that yields minimum mass flow-rate achieves a reduction of 46.22% on pressure pulsation, and 61.63% on mass flow-rate fluctuation. This discrepancy could possibly be attributed to simulation uncertainties since the mass flow-rate fluctuation values derived from two models are extremely close. This research demonstrates that an effective measure to improve the stability of flow fields is to increase the spacing of pipelines on both sides of the muffling segment, thus extending the time and distance of fluid movement and facilitating the sufficient and uniform flow of fluids inside the chamber. However, this could cause a decrease in the mean mass flow and this is a potential cost of the design. The effect on compressor work and COP would require a system-level model and is left for future work. While this study evaluates flow-side excitations only, acoustic quantities are not computed, and acoustic validation is left for future work.